Rechargeable batteries use polymer binders to bind the active particulate material together and adhere this particulate material to the current collector in the fabrication of battery electrodes. The binder is generally comprised of one or more polymers. The binders commonly used in commercial li-ion batteries are polyvinyledene fluoride (PVDF), ethylene-propylene and a diene (EPDM). These polymers are generally insoluble in water and, thus are dissolved in an organic solvent such as N-methyl pyrrolidone (NMP). The organic solvent additionally serves as a dispersion medium for the active materials. Some disadvantages of using organic solvents are that they have relatively high cost, can possess negative environmental impacts, and pose disposal issues. Further, PVDF is highly unstable and tends to break down at high temperatures.
Known water soluble binders, such as carboxy methyl cellulose (CMC), require a thickening agent to control the viscosity of the binder. Further, they exhibit only marginal adhesion capability. Polytetrafluoroethylene (PTFE) based water soluble binders also exhibit poor adhesion and do not exhibit good cycle life. Further, other known binders undergo hydrolysis under acid or basic conditions. To avoid the hydrolysis and to improve the dispersion, adhesion to the current collector, in a water-based blending process, the pH must therefore be tightly controlled.
Accordingly, there is a need for a water soluble polymer binder in rechargeable lithium batteries. This water soluble binder should exhibit stability throughout a wide pH range, which results in greater ease in preparing slurry for electrode fabrication.